How do I define my sexual orientation if I like a trans man?

There are a lot of binary⁠ (binary: Made up of two things or parts; a system with only two possible options or parts.) ideas and definitions happening in your questions. By that, I mean you're viewing things as only having two possible options, when really there are more than that. Let's tackle the question with the simplest answer first.

If you like a trans guy, then you like guys, because a trans guy is a guy. How that fits into your sexual orientation⁠ (sexual orientation: A term -- like homosexual, heterosexual, bisexual, queer, straight, lesbian, gay, asexual -- used to describe a person's usual or current pattern of emotional, romantic and/or sexual attraction to other people as it relates to or is about gender.) depends on who else you're attracted to. If you identify as bi, your feelings for him don't have to change that, since attraction to men falls within the usual definition of bisexuality.

It's not heterosexual⁠ (heterosexual: Someone who is only or mostly emotionally and sexually attracted to people of a different sex or gender than they are themselves.) to treat a trans guy as a guy. Heterosexuality is when someone is attracted only to people whose gender⁠ (gender: Characteristics that are seen or presented as distinguishing between male and female in a society. Gender may or may not include assigned or chosen: social roles, feelings, behaviors and/or presentation or appearance.) is different than their own (like men being attracted only to women). Treating a trans guys as a guy is just respectful. Doubly so if this is someone you love. That means using the right pronouns (he/him), the right name, and treating him as the expert on his gender.

As for how other people might treat you as his partner⁠ (partner: In a sexual context, a person with whom someone is having some kind of sex. The term "partner" can be used for all kinds of relationships, not just serious ones. "Partner" can also mean the person someone is with in a romantic or familial partnership.) , it's true that some might assume you're straight. But that may have less to do with him being trans and more to do with things like bi erasure, where people assume that a woman dating a man (or a man dating a woman) is straight. I do want to acknowledge that if you started dating this guy, there are people out⁠ (out: Short for ‘out of the closet’. When someone’s LGBTQ+ identity is known to other people.) there who would use your bisexuality as "proof" that he isn't really trans. After all, if you're bi, dating a trans guy could be an expression of your attraction to women right?

Wrong.

I've been in your situation, Rose, although I identified as straight when I fell in love with a trans guy. I found myself explaining over and over that no, my attraction to him did not change my sexual orientation because I liked guys and he was a guy. Sometimes the people who asked me that were genuinely curious, sometimes they were looking to invalidate one or both of us, and sometimes they were trying to get a rise out of him. However, it happening multiple times gave me a chance to identify the thought process behind the question, "But, like, does this mean you're gay⁠ (gay: A man who is attracted to other men, or a person of any sex or gender who is sexually and emotionally attracted to people of the same or a similar sex or gender. Often used alongside lesbian.) now?"

A lot of it comes down to people assuming that genitals⁠ (genitals: Body parts that are generally, culturally, scientifically or personally considered to be external sexual or reproductive organs.) are what determines gender.

When a person is born, they're assigned a sex⁠ (sex: Different things people choose to do to actively express or enact sexuality and sexual feelings; often this involves genitals, but not always. ) (most often male or female) based on the kind of genitals they have. Some people treat that assignment as the final word not only on a person's sex, but on their gender: men have penises, women have vaginas, end of story.

Other people understand that gender isn't just about your body; it's about how you think and feel, and how you express yourself. Your gender identity⁠ (gender identity: A person's own sense of whether and in what sense they feel they might be a man, a woman, neither, a mixture of genders, or another gender entirely.) is connected to how your culture and society categorizes gender--including how many genders there are and the traits of each one--and the ways in which you embrace or reject those categories.

But sometimes even people who grasp that gender is complicated believe, deep down, that the sex someone was assigned at birth is what really determines their gender. Which means they're right back to believing that genitals equal gender. However, they know it's rude to misgender⁠ (misgender: To assign someone a gender that isn’t their own, like by addressing someone with incorrect pronouns or directing someone to a bathroom that doesn’t match their gender identity.) trans folks, so they end up viewing a trans guy as almost a guy, but not quite, because he doesn't have a penis⁠ (penis: One of the two external reproductive organs of people often assigned male at birth.) . This same logic, which I see reflected in your question, can influence how they think of attraction. If someone is attracted to men, that means they're attracted to penises, which means their attraction to a trans guy is different than their attraction to men.

There are a few things wrong with that logic. For starters, some trans men have penises as a result of bottom surgery. More than that, when we first see someone and go, "Dang, they're hot," we're almost never looking at their naked genitals. Maybe we get to later, but it's not the thing that triggers our attraction to them. In other words, we're attracted to the gender someone presents to the world, not the sex they were assigned at birth. That's why being attracted to a trans guy falls under the category of "being attracted to men."

I should mention I'm assuming the guy you're interested is a binary trans person: someone who was assigned one of two sexes -- male or female -- at birth but whose gender is actually the sex (in this case, man or woman) they weren't assigned instead. But it's important to remember that there are lots of ways of being trans -- and lots of genders, period⁠ (period: The fluid – or flow – which contains blood, from the shedding of the uterine lining usually at the end of each menstrual cycle. Sometimes periods may be less fluid and more globby; this is the state that is often mistaken for blood clotting.) -- that fall outside of the male/female and man/woman binary.

As the diversity of genders becomes more visible and more people are able to be open about their identity⁠ (identity: The defining character or personality of an individual; who we feel like we are as a person.) , our chances of encountering a gender non-conforming person who we're attracted to increases. That, in turn, can cause us to question our sexual orientation. What does it mean if I'm attracted to a non-binary person? To people of any gender identity as long as they're masculine⁠ (masculine: Describes something society associates with or attributes to men and boys or a state, experience or assignment of being male.) in their gender expression⁠ (gender expression: The way people externally communicate gender identity to others through their behavior and their outward, chosen appearance.) ? To everyone except cis men?

The short answer is: you get to define your sexual orientation in whatever way feels right to you. I don't get to dictate your sexual orientation, just like no one gets to dictate mine. Like gender, sexual orientation is deeply personal, and the only way to know how someone identifies is to take them at their word.

That brings us to your question about identifying as pansexual⁠ (pansexual: Someone who may be/is attracted to other people of any gender or sex, not just men or women.) instead of bisexual⁠ (bisexual: A term for sexual orientation which either describes a person who can be sexually and emotionally attracted to people of more than one gender.) . Part of what may be tripping you up is there are some people who take the "bi" in bisexuality literally and insist it means bi people can only be attracted to two genders (men and women), and that pansexuality is for people who are attracted to more than two. There's also a weird strain of thought claiming that bi people can ony be attracted to cis men and women, which brings us right back to the conflating gender and genitals issue and treating trans men and women as another gender, rather than just one of the many ways a man or woman can be. Then there are people insisting pansexuality is the only identity that leaves room for attraction to non-binary individuals, ignoring the fact that plenty of bi folks date non-binary folks.

It all gets very messy and very shouty quickly, and my view on the matter can be summed up by this meme; the differences in labels are important to some people and not to others, and trying to generate the one, true definition of bisexuality or pansexuality that everyone agrees on is like nailing Jell-O to a tree.

I hope I've outlined some reasons why being attracted to a trans guy doesn't invalidate your bisexuality, but if you want to try other labels out to see how they feel, that's something you get to do. You're the boss of your sexual orientation, just like your crush is the boss of his gender identity. And if you approach each other with that mutual respect, you'll be just fine.

More like this:

• What's This Bisexual/Pansexual All About, Anyway?
• Dating and Gender Roles When One Partner is Trans
• The Rainbow Connection: Orientation for Everyone
• A Quick and Easy Guide to Queer and Trans Identities by Mady G. and J.R. Zuckerberg
• My Boyfriend is Trans, but Not Out Yet. How do I Support Him?
• Trans Summer School